Analysis Of Cfg Pile Composite Foundation Settlement Character Of High-speed Railway

At present, CFG pile composite foundation is widely used in Wuhan-Guangzhou, Zhengzhou-Xi'an passenger dedicated lines, Beijing-Tianjin intercity transit Beijing-Shanghai high-speed railway, etc. However, the theory of CFG pile composite foundation is lagged far behind practical engineering applications, in addition, the mechanism and calculation theory of CFG pile composite foundation under flexible foundation is not mature, and of no engineering experience support. From the present situation of high-speed railway and passenger dedicated line in China, it is very necessary to carry out the study of CFG pile composite foundation.Based on this, relying on Ministry of Railways'major S&T project (item number: 2007G046):"comprehensive technology study on CFG pile composite foundation in Beijing-Shanghai high-speed railway" sub task, the author studied the settlement characteristics of CFG pile composite foundation in north section of Beijing-Shanghai high-speed railway systematically by using geotechnical test in/out door, monitor test on the field, theoretical analysis, numerical calculation, Programming with advanced computer language, etc.Firstly, the author used drilling and various test methods on the test sections to study the strata feature comprehensively, including comparative analysis on results of various test methods, obtained some important achievements and conclusions which had favourable application value with similar projects in this area.Secondly, comprehensive monitoring, launched to the test section of CFG pile composite foundation, helped to gain a larger quantity of data, and on the basis of this, careful collation and analysis carried out to the data which obtained the displacement discipline of CFG pile composite foundation in medium compression stratum, and showed that surcharge preloading is necessary to decrease settlement. For this test point, under deep medium compression stratum and relatively good bearing stratum around CFG pile end, the overpressure standing time of high-speed railway roadbed could be cut down from at least 6 months, the current design requirement, to about 3 months, the research has an significant effect on project progress acceleration and economic benefits raise. Under the contrastive analysis of pile-net structure and pile-raft structure of CFG pile composite foundation, it showed that the overall characteristic of CFG pile-raft structure (settlement, horizontal deformation, load sharing ratio) was better than pile-net structure, because of its larger bearing capacity and smaller settlement; under the contrastive analysis of pile-raft structure of CFG composite foundation in section B, C and D, it is recommended to applied pile-raft structure in section D.Then, the author compared the deformation calculation methods of reinforced sections and subjacent bed of CFG pile composite foundation, analyzed theoretically the advantages and disadvantages of the methods, applied VC++language to organize calculation program based on Mindlin-Boussinesq combination theory; calculated the settlement of CFG pile composite foundation in test section, and compared and analyzed the results to other calculation theories and measured values, achieved some important conclusions. So it is recommended to apply Mindlin-Boussinesq combination theory to calculate settlement.Again, the author used PLAXIS 3D finite element software to simulate field test under various conditions, analyzed the influences of pile length, pile spacing, cushion to CFG pile composite foundation settlement, analyzed comparatively of the natural foundation and CFG pile composite foundation, and achieved some important conclusions in CFG pile composite foundation settlement and deformation characteristics. It is feasible and reasonable using PLAXIS 3D finite element model to analyze CFG pile-raft composite foundation settlement mechanism.Finally, procedures programming by MATLAB language based on XGM (1,1) model used to predict settlement was applied to predict the settlement of CFG pile composite foundation in test sections then compared with the measured settlement, they are consistent with each other. So it is favorable using XGM (1,1) model to predict settlement of CFG pile composite foundation.